The overall goal of this project is to study the regulation of E2F1 stability and specific role of E2F1 for DNA damage. While the function of E2F in cell cycle progression is well established, our recent works demonstrate an important role of E2F1 for DNA damage response. ATM/ATR can phosphorylate E2F1 and lead to stabilization of E2FI. The induction of E2F1 is required for DNA damage-induced apoptosis. Thus E2F1 appears to participate in the DNA damage checkpoint control which is often lost in human cancers. To further understand how E2F1 participates in the checkpoint control, we identified three proteins that interact with E2F1 and may regulate E2F1 during DNA damage. We propose to study how these proteins regulate E2FI. Aim 1: Explore the role of TopBP1/E2F interaction in transcription and replication control. To test the hypothesis: Regulation of E2F1 by TopBP1 is required for proper DNA damage response. Aim 2: Test the role of 14-3-3 in the regulation of E2F stability and activity. To test the hypothesis: Binding of 14-3-3 to E2F1 induces the stability and activity of E2F1. Aim 3: Test the role of hHYD for E2F degradation. To test the hypothesis : hHYD targets E2F1 for ubiquitination and binding of 14-3-3 protects E2F1 from degradation. We will employ biochemical, genetic and fluorescent microscope methodology as well as DNA microarray analysis to address these questions. The knowledge obtained from this study will be important in the understanding of how normal cells respond to genotoxic stress. A loss of the proper response may lead to _tumor develop.